1. Field of the Invention
The invention relates to the area of combustion technology. It relates to a main-burner arrangement of a combustion chamber, in particular a gas-turbine combustion chamber, which can be used both for premix burners and for partial-premix burners and for diffusion burners.
2. Discussion of Background
To broaden the operating range of a combustion chamber, a pilot burner is used in low load ranges, and the actual burner group, the main burners, are used under high load conditions. The present invention relates to the arrangement of the main burners of a combustion chamber.
Main-burner arrangements of gas-turbine combustion chambers in which the burners all have the same orientation and are arranged in a symmetrically distributed manner around the combustion-chamber axis are known. Here, the burner axis is generally arranged parallel to the combustion-chamber axis or the central sectional plane in the case of annular combustion chambers, but also known are arrangements in which the burner axis and the combustion-chamber axis are inclined relative to one another. In general, the direction of flow of the burners is axial with a slightly radial component.
To produce an additional swirl in the combustion chamber, tangentially inclined burner arrangements are also known.
Common to all the embodiments is a regular arrangement of the burners or, in the case of annular combustion chambers, a regular arrangement of at least all the burners of a ring.
This arrangement is chosen in order to achieve as homogeneous a distribution of the combustion gases as possible even in the primary combustion zone, this leading to homogeneous exit distribution of the combustion-chamber exhaust gases. However, this results in inevitable disadvantages.
Particularly in the case of premix burners, a homogeneous reaction zone, i.e. a homogeneous "flame front" tends towards unwanted pressure pulsations. Since the distance between the burners and the combustion zone is the same, all the burners have the same time constant. If a slight disturbance, which is always present due to turbulence, now occurs, all the burners respond with conversion fluctuations. If, in turn, these fluctuations take place at a suitable point in time, a pulsation with amplitudes that are deleterious to the machine can build up.
The excitation of the pressure pulsations can also be effected by other mechanisms, for example periodic separations and entropy waves among many other factors.
A further disadvantage of such a regular arrangement of the burners consists in that transverse ignition from burner to burner is made more difficult since the transverse ignition is in this case brought about purely by swirl-induced transverse flow.
EP 0 655 581 A1has disclosed a burner with at least one first hollow member, in which gaseous oxidizing agent flows along, and at least one second hollow member for the introduction of fuel into the flow of oxidizing agent, and, in this burner, the mouths of the second hollow members are arranged asymmetrically in relation to the first hollow member, such that the mixing zone upstream of the flame contains a region in which the ratio of fuel to oxidizing agent is substoichiometric. Various members, for example tubes, of different cross-sections can also be arranged asymmetrically in the first hollow member. With this arrangement, the power of the combustion chamber is increased, on the one hand, and the intention is, on the other hand, to reduce the NO.sub.x emissions. The solution proposed is based on the fact that mixing at the burner outlet is not homogeneous, there being regions with higher or lower concentrations of fuel than the average. However, if significantly lower NO.sub.x emissions are to be achieved, the air/fuel mixture must have been completely mixed, i.e. the solution presented in EP 0 655 581 A1 is not useable.
The Applicant is also aware of a method for operating a combustion chamber equipped with premix burners, the combustion chamber being fitted with burners of the same geometry and size which are misaligned relative to one another. These burners are operated with different air ratios .lambda. and therefore have different flame temperatures. This leads to a widening of the operating range of the combustion chamber by making possible stable operation of the combustion chamber even in the low load range without staggering the burners. The disadvantage is that a certain increase in the NO.sub.x emissions has to be accepted.
Another possibility for misaligning the burners is known from EP 0 686 812 A1. This discloses a method for operating a burner for a gas turbine, in which the fuel is introduced at different axial locations along the burner, e.g. even before the swirl blading, and the flow of the fuel to a downstream combustion zone is controlled in order to achieve asymmetrical flow of the fuel via the burner. This likewise leads to a widening of the operating range of the burner since, in the low load range, severe acoustic noise and resonance (pressure pulsations) are avoided and a reduction in the dynamic pressure loss without an increase in the NO.sub.x emissions is achieved. Here, however, the introduction of gas ahead of the swirl blading generates a combustible mixture ahead of the swirl generators. Experience has shown that such arrangements tend toward flashback with the risk that the blades will be burned away. The very complex construction of the fuel feed system is also disadvantageous.